1. Field of the Invention
The present invention relates to the field of threaded fasteners.
2. Description of the Related Art
Threaded fasteners, such as bolts or screws, have a variety of uses. Typically, a threaded fastener will have a threaded shank portion and an head. Torque is applied to the head to drive the threaded shank into threaded engagement with a nut or an other threaded surface.
To drive the fastener, the head may have recessed slots or grooves, such as a Phillips type, so that a tool may be inserted into the recess to apply torque to the fastener. Or, the head may have a polygonal periphery, so that a tool, such as a wrench or a socket, may engage the periphery of the head to apply torque to the fastener.
Often, the application of torque to the head can cause damage to head so that it becomes unusable. The slots, grooves, or periphery may "cam out" so that torque may no longer be applied to the head. This can occur if the tool is not properly seated when torque is applied. Or, it can occur if the fastener is over-tightened or if the threads become damaged, such as by galling, fusing, or corroding. Typically, the fastener is in threaded engagement when the head becomes unusable, so much time is lost removing the fastener. Yet another problem caused by improper seating, damaged threads, or over-tightening is that they can cause the tool to break rather than camming out the head.
Some head designs are better at preventing camming while others at protecting the tool. For example, the Phillips head or the head employing a polygonal periphery may cam out, while those with arcuate slots can cause the blades of the tool to break.
Moreover, it is often desirable to countersink a fastener flush with adjacent material to improve appearance or functionality. It is often desirable, therefore, to have the periphery of the head meet closely with the adjacent material. This is all but impossible when a periphery engaging tool is used because a large space must be left so that the tool may engage the periphery of the head while a portion of the head is screwed below the surface of the adjacent material. These type heads, therefore, are not desirable in countersink applications.
Providing the head of the fastener with a countersink also limits the depth of the recess near the periphery of the head. If the recessed area is too deep, it may extend through the countersink portion and allow the tool to contact the interior side of the adjacent material thereby interfering with operation of the tool. Or, if the recess is large and deep so that it approaches the countersink wall of the head, it may weaken the integrity of the head and allow the top portion of the head to separate from the rest of the head when torque is applied.